1. Field of the Invention
This invention relates to a thermoplastic hot melt adhesive for bonding metals. According to the present invention, a hot melt adhesive for metals is provided which exhibits an excellent adhesive strength at a low temperature and in a short period of time after application, and which has a long-lasting quality.
2. Description of the Prior Art
As thermoplastic hot melt adhesives in the form of films, there have been known polyethylene, ionomers, ethylene/vinyl acetate copolymer, polyamides (nylons), etc.
However, when polyethylene, ionomer or ethylene/vinyl acetate copolymer has been used for effecting the adhesion of metals, a sufficient adhesive strength has not been obtained and the durability of the adhesive bond has been unsatisfactory. Thus, it has been difficult to make practical use of such adhesives for metals. Further, polyamides (nylons) are known to be effective adhesives for metals. However, they have many disadvantages. Even when there is employed as an adhesive a homopolymer of nylon 11 or nylon 12, which are considered to have relatively low melting points, low water absorption and excellent durability, a hot melt application temperature of above 200.degree. C is required. Such a high temperature is undesirable because of its possible bad effects on the quality of the objects which are adhered together. Further, if the metal has an untreated surface or a surface cleaned or treated by the conventional cleaning or surface treatment methods employed in industry, such as etching, deoiling, etc., a sufficient lasting adhesive effect cannot be obtained. Accordingly, it is necessary to apply a primer of an epoxy resin, phenol resin or the like before applying the adhesive. This procedure is disadvantageous from the practical viewpoint.
Further, there have been proposed so-called "internally plasticized nylon copolymers" of lower melting points, which are synthesized by copolymerization of 2 or more of caprolactam, lauryl lactam, nylon 6-6 salt and nylon 6-10 salt. Although these nylon copolymers make it possible to bond metals to each other at a relatively low temperature, they also cause an increase in the water absorption and a reduction in the durability of the adhesive bond. There have also been proposed so-called nylon/epoxy film adhesives comprising an internally plasticized polyamide copolymer and epoxy resins, etc. Further, a film of nylon 11 or nylon 12 homopolymer can be coated with an epoxy resin or the like. However, those adhesives are expensive and they have a disadvantage in that a considerable curing time (generally longer than 30 minutes) is required for completing the hardening. Thus, they are unsuitable as a general purpose adhesive for metals.